Role of scattering in virtual source array imaging

We consider imaging in a scattering medium where the illumination goes through this medium but there is also an auxiliary, passive receiver array that is near the object to be imaged. Instead of imaging with the source-receiver array on the far side of the object we image with the data of the passive array on the near side of the object. The imaging is done with travel time migration using the cross correlations of the passive array data. We showed in [J. Garnier and G. Papanicolaou, Inverse Problems {28} (2012), 075002] that if (i) the source array is infinite, (ii) the scattering medium is modeled by either an isotropic random medium in the paraxial regime or a randomly layered medium, and (iii) the medium between the auxiliary array and the object to be imaged is homogeneous, then imaging with cross correlations completely eliminates the effects of the random medium. It is as if we imaged with an active array, instead of a passive one, near the object. The purpose of this paper is to analyze the resolution of the image when both the source array and the passive receiver array are finite. We show with a detailed analysis that for isotropic random media in the paraxial regime, imaging not only is not affected by the inhomogeneities but the resolution can in fact be enhanced. This is because the random medium can increase the diversity of the illumination. We also show analytically that this will not happen in a randomly layered medium, and there may be some loss of resolution in this case.Comment: 22 pages, 4 figure

Three-dimensional ambient noise modeling in a submarine canyon

Author Posting. © Acoustical Society of America, 2019. This article is posted here by permission of Acoustical Society of America for personal use, not for redistribution. The definitive version was published in Journal of the Acoustical Society of America 146(3), (2019): 1956-1967, doi:10.1121/1.5125589.A quasi-analytical three-dimensional (3D) normal mode model for longitudinally invariant environments can be used to compute vertical noise coherence in idealized ocean environments. An examination of the cross modal amplitudes in the modal decomposition of the noise cross-spectral density shows that the computation can be simplified, without loss of fidelity, by modifying the vertical and horizontal mode sums to exclude non-identical mode numbers. In the special case of a Gaussian canyon, the across-canyon variation of the vertical wave number associated with each mode allows a set of horizontally trapped modes to be generated. Full 3D and Nx2D parabolic equation sound propagation models can also be used to calculate vertical noise coherence and horizontal directionality. Intercomparison of these models in idealized and realistic canyon environments highlights the focusing effect of the bathymetry on the noise field. The absolute vertical noise coherence increases, while the zero-crossings of the real component of the coherence are displaced in frequency when out-of-plane propagation is accounted for.The authors wish to acknowledge Arthur Newhall for his technical support. This work was supported by the Office of Naval Research, Ocean Acoustic Code 322OA, under Grant Nos. N00014-15-1-2629 and N00014-17-1-2692, and by the Natural Sciences and Engineering Research Council of Canada's Research Chair, and Discovery Grant program.2020-03-3

Evolution Of Water Reservoirs In The Early Solar System Through Their Oxygen Isotopic Composition

A new technique has been developed to enable analyses of δ170 and δ180 from small water samples extracted from meteorites. Isotopic measurements are made using a continuous-flow, isotope ratio mass spectrometer (Delta C, Finnigan Mat) utilising a helium carrier gas. Oxygen is extracted from water using the powdered, solid fluorinating agent CoF3, and is purified using a GC column (PLOT, 5Å). Reproducibility of the method, determined using solid standards and reference waters is ±.30%c for δ18O and +0.14%c for ΔI7O (1σ in each case). The technique has been applied to three suites of meteorites: Carbonaceous chondrites (CI and CM), SNC meteorites (four meteorites) and a selection of Antarctic samples, three eucrites and one ordinary chondrite. Results were used to constrain models of the origin of Solar System water and its subsequent interaction on the meteorite parent bodies. Results from carbonaceous chondrites suggest that water accreting to bodies in the early Solar System was enriched in both 17O and 18O and had a Δ170 of at least +2%c The isotopic composition of water evolved during hydrothermal alteration on parent bodies toward lower Δ17O values, more similar to that of the host rocks. The complex release profile indicate that these meteorites experienced several periods of hydrothermal activity. SNC sample results have suggested the presence of two isotopically distinct reservoirs, the silicate crust and the hydrosphere each possessing distinct Δ170 values. A third component with a large 17O excess (+4%) was detected in ALH84001. The magnitude of this anomaly indicates an origin due to heterogeneous accretion or late stage veneer and cannot be produced by hydrodynamic escape. Results from the eucrites and ordinary chondrite also suggested isotopically distinct parent body silicate and hydrous reservoirs and may also be consistent with widespread late input of isotopically distinct water in the Solar System

Carbon Chemistry Of Giant Impacts

Impact diamonds were found in several impactites from the Ries crater, Germany including fallout and fallback (crater fill) suevites. a glass bomb, impact melt rock and shocked gneiss. These diamonds formed two distinct grain size populations: 50-300?m apographitic. platy aggregates with surface ornamentation and etching that were observed using optical and scanning electron microscopy and 5-20?m diamonds which displayed two different morphologies identified using transmission electron microscopy and selected area electron diffraction. These 5-20?m grains comprised apographitic. platy grains with stacking faults, etching and graphite intergrowths together with elongate skeletal grains with preferred orientations to the individual crystallites. Thermal annealing of stacking faults and surface features was also detected. Stepped combustion combined with static mass spectrometry to give carbon isotopic analysis of individual diamonds. graphite and acid-residues indicate that the primary carbon source is graphite. This graphite was found to be 13C depleted with respect to similar samples from the Popigai impact crater. The admixture of presumably carbonate derived carbonaceous material is suggested to account for the 13C-enriched ?13C compositions encountered in whole-rock suevites known to include carbonate melts. On the basis of morphology. mineralogical associations diamond/graphite ratios and carbon isotopic compositions three possible formation mechanisms for impact diamonds are suggested: fast. high temperature conversion of graphite following the passage of the shock wave, a vapour phase condensation or growth within substrate minerals or an orientated stress field and the incomplete transformation of a mixture of amorphous and crystalline graphite. Further more exotic mechanisms such as intermediary carbyne phases cannot be discounted. Impact diamonds, 1-5?m in size. were also identified in suevite residues and a black matrix lithic breccia from the Gardnos impact crater. Norway. The carbon isotopic compositions are in agreement with previous measurements of whole rock samples with a small 13C-enriched component probably representing diamond

Protocol Layering and Internet Policy

An architectural principle known as protocol layering is widely recognized as one of the foundations of the Internet’s success. In addition, some scholars and industry participants have urged using the layers model as a central organizing principle for regulatory policy. Despite its importance as a concept, a comprehensive analysis of protocol layering and its implications for Internet policy has yet to appear in the literature. This Article attempts to correct this omission. It begins with a detailed description of the way the five-layer model developed, introducing protocol layering’s central features, such as the division of functions across layers, information hiding, peer communication, and encapsulation. It then discusses the model’s implications for whether particular functions are performed at the edge or in the core of the network, contrasts the model with the way that layering has been depicted in the legal commentary, and analyzes attempts to use layering as a basis for competition policy. Next the Article identifies certain emerging features of the Internet that are placing pressure on the layered model, including WiFi routers, network-based security, modern routing protocols, and wireless broadband. These developments illustrate how every architecture inevitably limits functionality as well as the architecture’s ability to evolve over time in response to changes in the technological and economic environment. Together these considerations support adopting a more dynamic perspective on layering and caution against using layers as a basis for a regulatory mandate for fear of cementing the existing technology into place in a way that prevents the network from innovating and evolving in response to shifts in the underlying technology and consumer demand

Protocol Layering and Internet Policy

An architectural principle known as protocol layering is widely recognized as one of the foundations of the Internet’s success. In addition, some scholars and industry participants have urged using the layers model as a central organizing principle for regulatory policy. Despite its importance as a concept, a comprehensive analysis of protocol layering and its implications for Internet policy has yet to appear in the literature. This Article attempts to correct this omission. It begins with a detailed description of the way the five-layer model developed, introducing protocol layering’s central features, such as the division of functions across layers, information hiding, peer communication, and encapsulation. It then discusses the model’s implications for whether particular functions are performed at the edge or in the core of the network, contrasts the model with the way that layering has been depicted in the legal commentary, and analyzes attempts to use layering as a basis for competition policy. Next the Article identifies certain emerging features of the Internet that are placing pressure on the layered model, including WiFi routers, network-based security, modern routing protocols, and wireless broadband. These developments illustrate how every architecture inevitably limits functionality as well as the architecture’s ability to evolve over time in response to changes in the technological and economic environment. Together these considerations support adopting a more dynamic perspective on layering and caution against using layers as a basis for a regulatory mandate for fear of cementing the existing technology into place in a way that prevents the network from innovating and evolving in response to shifts in the underlying technology and consumer demand

On the use of the finite element method for the modeling of acoustic scattering from one-dimensional rough fluid-poroelastic interfaces

textA poroelastic finite element formulation originally derived for modeling porous absorbing material in air is adapted to the problem of acoustic scattering from a poroelastic seafloor with a one-dimensional randomly rough interface. The developed formulation is verified through calculation of the plane wave reflection coefficient for the case of a flat surface and comparison with the well known analytical solution. The scattering strengths are then obtained for two different sets of material properties and roughness parameters using a Monte Carlo approach. These numerical results are compared with those given by three analytic scattering models---perturbation theory, the Kirchhoff approximation, and the small-slope approximation---and from those calculated using two finite element formulations where the sediment is modeled as an acoustic fluid.Mechanical Engineerin